1. Field of the Invention
The invention relates to the field of acoustic gas monitoring and, more particularly, to the in-line monitoring and control of the composition of gas mixtures.
2. Description of the Related Art
In many manufacturing operations, accurate information concerning a reaction gas composition is necessary to control a particular process. For example, chemical vapor deposition (CVD) processes require precise gas mixtures to reliably create materials of a specific composition. Formation of semiconductor materials and optical fiber performs often involves incorporation of dopant materials in very small concentrations. The dopant material is supplied by a dopant precursor gas which is mixed with other deposition gases in a reaction chamber. Because of the low concentration used in the vapor deposition process, the dopant gas is usually mixed with a carrier gas to ensure an even distribution of dopant within the reaction chamber. The career gas must deliver a consistent quantity of the dopant gas. In-line gas monitoring is often used to ensure this consistent delivery.
Acoustic monitoring of gases can employ ultrasound, i.e., sound waves having a frequency ranging from a few kHz to 10 MHz. Acoustic techniques have been extensively used for gas flow monitoring. More recently, efforts have turned to developing acoustic cells and processes which can determine the concentration of a component of a binary gas mixture. In general, acoustic concentration analysis of a gas mixture is performed by measuring the speed with which sound waves propagate through a gas mixture. Because the speed at which the sound waves travel through a gas is related to molecular weight, the concentration of a component of a gas mixture can be accurately determined.
For a single component system under ideal conditions, the velocity of sound, V.sub.s can be obtained from the following equation: ##EQU1##
where .gamma. is the specific heat capacity ratio (C.sub.p /C.sub.v), R is the universal gas constant (8.3143 J/mol K.), T is the absolute temperature in degrees Kelvin, and M is the molecular weight of the gas in kg.
In the case of a binary gas mixture, a similar relationship exists, with .gamma. and M replaced by .gamma. and M. The acoustic velocity of a binary gas mixture is then represented by: ##EQU2##
where .gamma. is the average specific heat capacity ratio given by: ##EQU3##
and M is the mean molecular weight of the binary gas mixture given by: EQU M=(1-x)M.sub.1 +xM.sub.2
where x is the mole fraction of a second gas and M.sub.1 and M.sub.2 are the respective molecular weights of the first and second gases.
To solve for the concentration of a gas component, x, a quadratic equation is formulated from the above equations: ##EQU4## Where: A=constant=RT
a.ident..gamma..sub.2 and b.ident..gamma..sub.1, PA1 c.ident.M.sub.2 and d.ident.M.sub.1. PA1 x=concentration (mole fraction) of species corresponding to parameters a and c PA1 (1-x)=concentration (mole fraction) of species corresponding to parameters b and d PA1 V.sub.s =is in units of meter/second.
This equation is solved for x using the quadratic formula. Thus, the measurement of the velocity of sound through a binary gas mixture yields the relative amounts of the two gas components.
The principle of acoustic gas analysis has been used in a gas monitoring cell shown in published UK Patent Application GB 2,215,049, the disclosure of which is incorporated by reference herein. In the disclosed cell, ultrasonic pulses are generated by an ultrasonic transducer. The transducer is composed of a piezoelectric material, such as lead zirconate titanate, and is positioned opposite a second transducer. The transit time of sonic pulses between the transducers is measured and used to yield the sound velocity. From the velocity, the composition of the binary mixture is determined.
In the cell of the U.K. patent application, metal gaskets are employed for gas sealing. Because these metal gaskets permit acoustic coupling through the body of the cell, each transducer is supported on an array of mounting pins to minimize acoustic coupling between the transducer and the cell body.
Although the cell of the U.K. application reduces acoustic coupling, there is still sufficient extraneous noise to interfere with the acoustic measurement process. The result is a loss of sensitivity of the cell. Additionally, the cell of the U.K. application operates using ultrasonic frequencies on the order of one megahertz. In general, as sound frequency increases, the attenuation of sound waves also increases. At frequencies in the megahertz range, attenuation of sound in the gas being analyzed is a problem, particularly when attempting to measure high sound absorptive gases, which absorb ultrasound in higher ultrasonic frequency ranges. Because the cell of the U.K. application has a short path length, higher frequencies are required to attain the resolution needed to detect the arriving pulse.
U.S. Pat. No. 5,392,635 to Cadet et al. suggests that the transducer be acoustically isolated to solve the problem of unwanted acoustic coupling. FIG. 1 is a partial cross-section of the acoustic analysis gas cell 10 described in Cadet et al. The transducer 80 is placed opposite the end 22 of conduit 20. Gas flows through a gas port 70, through a passage 35 and a cylindrical gap 39 and into the conduit end 22. The gas passes transducer 80 as it flows into the conduit end 22. As illustrated in FIG. 1, Cadet et al. couple the transducer 80 to a layer of acoustic isolation material 82. The acoustic isolation material 82 is an elastomeric material such as a silicone elastomer. Cadet et al. also propose inserting a layer of this elastomeric material 83 between the transducer 80 and the gaseous mixture to be analyzed. The acoustic isolation provided by Cadet et al. produces a signal-to-noise ratio of at least 4:1. However, since the elastomeric material is exposed to the gas being analyzed, the elastomeric material must withstand exposure to corrosive gas environments. Since elastomers typically degrade when subjected to corrosive gases, the solution offered by Cadet et al. may not be suited for acoustic analysis of all gas mixtures.
In U.S. Pat. No. 5,060,506, a method and apparatus are disclosed for monitoring the ratio of gases in a two-gas mixture using ultrasound. The transmitter used to generate the ultrasonic pulses is excited with a signal having a plurality of successive bursts, each of which includes a preselected number of excitation pulses at the resonant frequency. The initial pulse in each burst is separated from the final pulse in the preceding burst by a quiescent time period of sufficient duration to assure dissipation of transients so that standing waves do not form.
There is a need in the art for improved acoustic cells and methods for analyzing the composition of gas mixtures. More particularly, there is a need in the art for acoustic cells which are compatible with vacuum systems and corrosive gas environments without acoustic coupling of the transducer to the cell body. Additionally, there is a need in the art for an acoustic gas composition analysis cell which operates in a frequency range which permits measurement of a wide variety of gas mixtures.